I tried charging the Optima with my Battery Tender, but it still wouldn't start the genset.
So I drained the water seperator (though it doesn't look at all like what the manual shows..)
Then, before I went out and bought new batteries, I decided to hook up my Harbor Freight 12V Jump Starter in place of the dead battery, and it worked. Started right up.
So now I'm thinking it may be cheaper @ $80 and more versatile to just buy another HF Jump Starter, keep them charged in the house, and hook them up to the generator when I need to start it.